242 APPLIED SCIENCE 



Very little is yet known as to the cause of earth-currents or 

 their laws. The electro-motive force producing them does not 

 seem to increase with the distance between the two ends of the 

 line after the first few miles. No greater force than that due to 

 ten ordinary Daniell's cells is reported by Sir W. Thomson and 

 Mr. Varley to have been observed at any time between the two 

 ends of the cable. Much the same may be said of the Malta- 

 Alexandria Cable ; but Mr. Varley has spoken of a force equal 

 to 400 cells on short land lines in England. Thus submarine 

 lines appear to have in this respect an advantage over air lines. 

 The latter are further subject to induction from changes in the 

 atmosphere, producing effects similar to earth-currents. The 

 conducting mass of the sea should screen submarine circuits 

 from all these effects ; but Mr. Varley has informed the writer 

 that currents were observed from the Atlantic which seemed to 

 be of this nature. One most singular phenomenon was also 

 communicated to the writer by Sir W. Thomson and Mr. 

 Varley. Owing to changes in the potential of the sea, the 

 capacity of the cable for a statical charge varied. The immense 

 Leyden jar formed by the cable at times therefore poured out 

 a current at one end, while the other was insulated, giving 

 apparently more than infinitely good insulation. Not only did 

 the battery used to test insulation then fail to force any current 

 through the gutta-percha of the insulated cable, but a current 

 was actually forced back on the battery, as if coming through 

 the gutta-percha into the cable. From this cause, even if the 

 Atlantic Cables were joined in a metallic circuit, continual 

 currents would fluctuate to and fro between them, owing to 

 changes in the difference of potential of the sea in their respective 

 tracks, thirty miles apart. The two cables afford an unrivalled 

 opportunity of studying earth -currents, about which really little 

 is known ; and it is to be hoped the opportunity will not be 

 neglected. There is not the slightest reason to fear that they 

 will prove any obstacle to the transmission of messages through 

 any submarine cable, of whatever length it may be. One method 

 of avoiding all disturbance from earth-currents is to use so 

 powerful a battery as to overpower their effects ; but this plan is 

 not to be recommended, since the action of a powerful bath TV 

 has been known to change small faults into great ones, and 

 though not even a small fault is believed to exist in either 



